mmg_233_2013_genetics_genomicswikiaorg-20200214-history
Horizontally acquired divergent O-antigen in the classical bordetellae
Inter- and intra-species horizontal gene transfer (HGT) provides a mechanism for increasing genetic diversity through the efficient, and rapid exchange of genetic material. The study by Hester et al. (2013) describes the HGT mechanism in the classical bordetellae as it relates to variation in the prominent O-antigen. (1) Evolutionary advantages have likely selected for variants of the O1- and O2-serotypes which elicit a poor immunogenic response in hosts. Background There are three closely related subspecies of Bordetella: B. bronchiseptica, B. parapertussis and B. pertussis. Infection from these pathogenic organisms can be asymptomatic or severe enough to cause lethal pneumonia. B. parapertussis, and B. pertussis are the causative agent of Whooping Cough in humans. (2) These organisms have diverged through a variety of mechanisms, including HGT, allowing each to thrive in their respective niche environment. Though examples of HGT of Bordetella genes related to environmental survival have been described, no known examples of HGT have been identified in genes related to differential infection abilities. These latter variations have been attributed to varying expression levels of shared virulence factor genes. O-antigen is a component of the lipopolysaccharide (LPS) Gram-negative factor that helps shield against the host immune system. A high level of O-antigen variation across species of bacteria has been described. While the O-antigen locus of most of the strains of B. parapertussis and B. bronchiseptica contain 24 genes, one B. bronchiseptica strain (MO149) contains a mere 15 genes which have considerable variation from the more common forms of the respective genes. Given the observed poor cross-reactivity of antibodies against O-antigen serotypes, HGT within this gene locus is an important area of focus for anticipating and treating classical Bordetellae infections. Study design Individual O-antigen gene alignments were created with MEGA5 software. The PARRIS application in Hyphy was used to determine site-specific selection within genes. Nucleotide sequences of the O-antigen in eleven diverse strains of classical Bordetella were aligned based on B. parapertussis strain 12822 using the program SSaha v2.2.1. Recombination breakpoint analysis was conducted using Recombination Detection Program (RDP3) and Genetic Algorithm for Recombination Detection (GARD). Because mobile elements associated with HGT were not observed, multiple software analysis protocols were employed to detect breakpoints, including: MaxChi, Chimaera, SiScan and GENECONV.All predicted recombination events were confirmed by at least six methods. In vitro and in vivo phenotype studies were then conducted to elucidate evolutionary pressures that might explain the observed genetic variation within the O-antigen of bordetellae. Results In this study the authors report evidence for significant variation in the O-antigen of bordetellae due to HGT through recombination. Interestingly, the study's finding indicate considerable loss of O-antigen gene regions in some strains with minimal to no new gene uptake. This is likely indicative of evolutionary pressure promoting fewer host-recognizable antigens, as well as the capacity for different strains to be present within a host at the same time without the occurrence of cross-reactivity. References 1. Hester SE, et al. (2013) Horizontally acquired divergent O-antigen contributes to escape from cross-immunity in the classical bordetellae. BMC evolutionary biology 13:209. 2. Diavatopoulos DA, et al. (2005) Bordetella pertussis, the causative agent of whooping cough, evolved from a distinct, human-associated lineage of B. bronchiseptica. PLoS pathogens 1(4):e45. 3. http://en.wikipedia.org